Electron withdrawing groups determine acidity-- but how do you know which groups are electron withdrawing?
Answer:
1) Electronegative elements.
2) Large positive oxidation states.
3) Positive charge on the connecting atom.
4) 1,3-shift resonance contributors swapping negative charge out of or postive charge into the reaction center.
You must consider both the sigma (inductive) and pi (resonance) contributors. On a benzene ring chlorine is sigma-withdrawing and deactivating but pi-donating and ortho,para-directing. [SbF5]- is a tremendously weak nucleophile, but [Sb2F11]- is much weaker still - same charge, many more delocalizations. The respective superacids are then incredibly strong and incredibly stronger.
level 2
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